Energy consumption monitoring: KPIs and how to start

Why measure per line and not per bill

The energy invoice arrives once a month, for the whole plant. It says how much was spent, not which line, which shift or which item weighed most: management control splits the total by estimate, and the production manager has nothing to act on. Besides, a rising consumption can have two opposite causes, because more was produced or because production got worse, and the total alone cannot tell them apart.

Measuring per line and per shift fixes both problems. It gives a consumption that can be compared with yesterday's, with the next line's or with the same item last month, and it finds the waste that has nothing to do with production: machines left on overnight, leaking compressed air, a plant starting earlier than needed. It is the first step for any saving action, because you cannot improve what you do not measure.

Mapping consumption: which meters to read

You do not need to measure everything from day one. Start from the map of measurement points, from the main meter down to the departments, and add meters where consumption is large or little known. In order of priority:

• Main electricity meter: the plant total, used to reconcile with the invoice.
• Sub-meters or power analysers on the panel of each line or department: energy (kWh), power (kW) and, if needed, power factor. They almost always have a Modbus TCP or Modbus RTU output.
• Other carriers: gas, steam, water and compressed air, where they weigh on a process's cost. Each has its own meter and unit (Sm³, m³, kg).
• Production: parts counted by the PLC or orders from the ERP, over the same interval as the consumption. Without production kWh remain a total; with production they become an indicator.
• Shift calendar: to separate consumption while machines produce from consumption while they stand idle.

The practical step is to read the meters with a node near the panel and bring the values to the same place as the parts produced. With Muvia Industrial the edge node reads power analysers over Modbus TCP and part counters from the Siemens S7 PLC with the drivers; meters with a serial-only output need a serial-to-Modbus TCP converter, because Muvia's drivers are for TCP networks. The differences between the protocols are in the guide to Modbus and OPC UA.

Energy KPIs (kWh/part, baseline, EnPI)

A few indicators cover almost everything. The examples refer to a moulding line over an 8-hour shift, with 729 parts produced (the same as in the OEE guide) and 1,458 kWh drawn; they are example data, not a customer's.

The baseline (in ISO 50001, the energy baseline) is the reference consumption the present is compared with: usually a twelve-month period, to include the seasons. The EnPI (Energy Performance Indicator) is the number the organisation chooses to follow the result: it can be a value (the month's kWh), a ratio (kWh per part) or a statistical model.

The model is needed when the simple ratio misleads. If part of the consumption is fixed, that is, does not depend on parts, kWh/part improves by itself when more is produced and worsens when less is, without anyone having done anything. With a model a + b × parts (here a = 150 kWh of fixed consumption per shift and b = 1.9 kWh per part, obtained from a regression on the reference year) you calculate the energy expected for the parts made and compare it with what was measured: 1,535.1 expected against 1,458 measured is 77.1 kWh saved, 5.0%. The comparison stays correct even if the volume changes.

ISO 50001 and the energy audit: what they ask for

They are two different things, and confusing them is the most common mistake. The energy audit (diagnosi energetica) is a legal obligation for some companies; ISO 50001 is a voluntary standard for an energy management system.

The audit is regulated by article 8 of Legislative Decree 102/2014 (in Italian). According to the text, large enterprises carry out an energy audit at their production sites in Italy, performed by energy service companies or energy management experts, the first by 5 December 2015 and then every four years. Energy-intensive companies, that is those covered by the Ministry of Economic Development decree of 21 December 2017, have the same obligations regardless of size. Large enterprises that adopt a system compliant with ISO 50001 are not subject to the four-year periodicity, provided the system includes an audit compliant with Annex 2 of the decree. Audits are submitted to ENEA. Whether a company falls into one of these cases, and with what deadline, must be checked against the current text and with ENEA: the rules change and this guide does not replace a consultant.

ISO 50001 asks for a cycle of continuous improvement, and measurement is the heart of the cycle. In practice, this is the list of what an auditor expects to find:

• Energy policy signed by management, with the commitment to improve energy performance.
• Energy review: the analysis of where and how energy is used, with the period's consumption and the expected one.
• Significant energy uses: the few plants and processes that weigh most, chosen on data and not by feel.
• Baseline and EnPI for each, with the variables that influence them (volumes, product mix, weather) and the normalisation method.
• Objectives and action plan with owners, deadlines and resources.
• Measurement plan: which quantities are measured, with which instruments, how often, and how they are kept calibrated.
• Internal audit and management review, with the evidence: consumption, deviations, closed actions.

Consumption crossed with production

A line's consumption becomes an indicator only when crossed with what the line has done. Three data sets are brought together: kWh per interval from the meter, parts counted by the PLC, and the order in progress, which comes from the ERP. With these three you calculate specific consumption per shift, per order and per item, and you see that the same item costs 2.0 kWh per part on one line and more on another, or that a changeover moves consumption more than the production hour it removed.

The same crossing feeds the rest: kWh per part enter the production cost per item, and the month's consumption enters the sustainability report. The complete path, from the Modbus TCP meters to the monthly PDF report, is in the use case on energy and consumption per line and per part.

Alarms on abnormal consumption

The dashboard says what happened, the alarm says when to step in. Consumption lends itself to a few very effective alarms, all based on comparison with what was expected and not on a fixed threshold:

• Draw with the line stopped: power exceeds the usual base load while the line is not producing. It is the cheapest signal to implement and the most rewarding.
• Specific consumption off baseline: the shift's kWh per part exceed the baseline by a chosen percentage, for example because of wear, a leak or an off-standard cycle.
• Power peak: the draw exceeds the agreed value, when a penalty or a contractual band depends on the peak.
• Missing data: the meter has not sent anything for an hour. A consumption that disappears is also a monitoring failure.

Muvia's alarms compare a value with a threshold and write every event in a register with acknowledgement, so that a record remains of who responded and when. To avoid noise, the threshold should be compared with the same shift and the same item, not with a single value for the whole day.